Reciprocal imaging systems are known such as, for example, the system described in the publication entitled "Reciprocal Optical System for Measuring the State of Focus of Reflected Images", Journal of the Optical Society of America, Vol. 58, No. 12, p. 1607, December 1968, by H. D. Crane and G. L. Pressman.
This prior art system involved imaging an illuminated pattern onto a reflecting surface and reimaging the reflection back through the same optical system onto the original pattern. If the first image is in focus on the reflecting surface, then by optical reciprocity, the returned image is congruent with and superimposed upon the original source pattern. As the reflecting surface moves out of the image plane, some of the returned light falls beyond the input pattern. Thus, intercepted light is nominally zero when the mirror is in the image plane, and increases with displacement in either direction away from focus. In practice there are serious problems arising from the need to separate the intercepted light from the projected light (that which will form the wafer image) and other sources of stray light.